1. Field of the Invention
This invention relates in general to current controllers and in particular to a predictive digital current controller for switching power converters.
2. Statement of the Problem
Switching power converters exploit the energy storage characteristics of magnetic and capacitive circuit elements. Therefore, the switching regulator takes discrete packets of energy from an input power source, stores the energy in a magnetic field of an inductor or as an electric field in a capacitor, and then transfers the energy to the load. The switching power converter takes an input current (AC or DC) from a voltage (or current) source and produces a voltage (or current) at the output that is different from the input voltage (or current).
The most common control technique for transferring energy from the energy storage element to the load is pulse width modulation (PWM). The packets of energy removed from the input power source are varied in duration, within a fixed operating period, as necessary to maintain an average energy transfer. Switching power converters exhibit high efficiencies because the power-switching elements are ideally lossless: when a switch is on, the voltage across it is very close to zero; when a switch is off, the current through the switch is very close to zero; either way, the product of voltage and current (i.e., the instantaneous power) across the switch is close to zero and, therefore, the switch is almost a lossless component; yet it is capable of controlling the energy transfer from input to output One method of controlling the feedback loop and the regulation characteristics of a switching power converter is current control.
A simplified schematic diagram of a boost converter is shown in FIG. 1. In general, a switching power converter, such as the boost shown in FIG. 1, consists of switches such as S1 and D1 in FIG. 1 and energy-storage components such as L1 and C in FIG. 1. The converter operates by turning one or more of the switches on or off. The on/off states of the switches determine how the energy-storage components, the input, and the output are connected. The length of time the switches are in on or off positions is the control variable that affects the conversion properties of the switching power converter.
To construct a voltage regulator, a feedback loop is constructed around a converter: the output voltage is sensed and compared to a reference; the error between the sensed voltage and the reference is amplified by an error amplifier (also called compensator); and the output of the compensator is the command to a pulse-width modulator that generates a pulsating waveform (or waveforms) that control the on/off state of the switches. In steady state, the output voltage is forced to be equal to or very close to the reference, in spite of possible load variations, input voltage variations, or component tolerances.
Two most popular implementations of the generic feedback loop description above are (1) voltage-mode PWM control, where only the output voltage is sensed and the compensator in the controller operates solely based on the error between the sensed voltage and the reference; and (2) current-mode control, where, in addition to the output voltage sensing, a current from the converter circuit is also sensed, and the compensator operates based on both the sensed voltage error and the sensed current. Advantages of the current-mode control include improved and more robust control performance, built-in over-current protection, the ability to enable current-sharing in paralleled converter modules, and/or the ability to control the current waveshape in applications that require this feature (such as in AC-DC rectifiers with power factor correction (PFC)). Furthermore, there are several variations on the current-mode control technique of which peak current mode control and average current mode control are the most popular. State-of-the-art implementations of voltage-mode PWM and current-control mode are explained in detail in Erickson, R. W., Maksimovic, D. Fundamental of Power Electronics, 2nd edition, Kluwer Academic Publishers, 2000.
Current programmed control finds wide applications in switching power converter applications. Current control can be classified as peak, valley, or average current control, depending on whether the maximum, the minimum, or an average point of the sensed inductor current is compared to the reference current. The fact that the inductor current is tightly controlled results in simpler converter dynamics, allowing simple and robust wide-bandwidth control. In addition, the peak current control offers fast over-current switch protection.
Given the fact that the switch (or the inductor) current is a fast-changing waveform, and that switching frequencies are in the range of hundreds of kilohertz (KHz) to megahertz (MHz), a direct implementation of the analog current programmed control in digital hardware is not easy. The need for a very fast analog-to-digital (A/D) converter to produce multiple samples of the sensed current per switching period, and the corresponding need for large signal processing capabilities, may require excessively complex hardware and complex algorithms. Sampling and processing result in a delay that can compromise control performance, especially in high-frequency applications. One way to improve the digital control performance is the predictive technique, which has been applied in three-phase systems. In one switching period, the duty cycle for the next switching cycle is calculated based on the sensed or observed state and input/output information, such that the error of the controlled variable is cancelled out or minimized in the next cycle or in the next several cycles. In particular, valley current control has been applied to DC-to-DC converters. It has been found that the “period-doubling” oscillation issues, which are notable in analog current programmed control, also exist in digital predictive current controllers.
Predictive techniques have found applications in single-phase rectifiers and DC-to-DC converters. However, the predictive technique has only been applied to the control of the inductor valley current using trailing-edge pulse width modulation. Valley current control using trailing-edge pulse width modulation has been described in S. Bibian and H. Jin, “High Performance Predictive Dead-Beat Digital Controller for DC Power Supplies”, IEEE Applied Power Electronics Conference, 2001 Record, pp. 67–73; and S. Bibian and H. Jin, “Digital Control with Improved Performances for Boost Power Factor Correction Circuits”, IEEE Applied Power Electronics Conference, 2001 Record, pp. 137–143.
In other applications, peak current control and average current control is preferred to valley current control. However, oscillations occur under operating conditions wherein the duty cycle is greater than 0.5 when trailing-edge pulse width modulation is used to predict a next duty cycle under peak current control and average current control. This is the same instability problem as in analog current control, where the instability is usually suppressed by adding a ramp signal to the sensed current signal. Under trailing-edge pulse width modulation, only predictive valley current control can be achieved for all operating conditions without oscillations.
For these reasons, a need exists for digital current programmed control technique that is effective for peak, average, and valley current control without complex hardware or complex algorithms.